1. Field
The invention relates to digital communications. More particularly, the invention relates to discontinuous transmission (DTX) detection.
2. Description of the Related Art
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems. For example, a CDMA system provides increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) xe2x80x9cTIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), (2) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Project 2xe2x80x9d (3GPP2) and embodied in a set of documents including xe2x80x9cC.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d the xe2x80x9cC.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d and the xe2x80x9cC.S0024 cdma2000 High Rate Packet Data Air Interface Specificationxe2x80x9d (the cdma2000 standard), and (4) some other standards.
Wireless telephone systems are capable of carrying both voice and data over the allocated communication channels. Digital wireless telephone systems are particularly suited to carrying data over the allocated communication channels. It is possible for the system to dedicate a channel to a user, via the user""s Mobile Station (MS), in order to accomplish data transmission. A continuously active channel is preferable when the anticipated data transmission is continuous. With a continuously active channel, the user is able to efficiently transmit or receive a continuous data stream over the allocated active channel of the communication system. However, the exploding increase of packet data applications, such as those used when communicating over the Internet, make allocating a continuously active channel to a single user an over allocation of resources. Control signals sent from a base station to a mobile station or messaging may occur only infrequently and it may not be desirable to allocate a continuously active channel for a limited purpose. Additionally, because wireless telephone rates are often tied to connection times, a user may not be willing to use a MS to connect to a remote network if a continuous active channel must be dedicated to the connection.
The designers of wireless telephone systems have recognized the desire for packet data applications over wireless channels. The designers have also recognized that packet data and the associated burst transmissions may be transmitted over channels that are not continuously active, but rather, allow for discontinuous transmission (DTX).
With discontinuous transmission, communication to a receiver over a channel does not occur continuously but may be cycled on and off. The receiver is then faced with the problem of determining when a signal was transmitted or when there is a lack of a signal. A receiver may easily make the DTX detection decision under ideal channel conditions. The receiver would demodulate the transmitted signals as they arrive and realize that lack of a signal to demodulate indicates that the transmitter is engaging in DTX, and is in fact not transmitting a signal.
However, a real world communication link does not operate under ideal channel conditions, nor does a real world receiver operate with an ideal demodulator. In a real world application, signal multipath, fading, path loss, noise, and interference corrupt the signal incident on the receiver. Additionally, the receiver demodulator is not ideal and may not accurately demodulate every received signal. The result of non-ideal characteristics is that a receiver demodulating a continuously transmitted signal will occasionally be unable to recover the transmitted data.
As an example, within any cell of a CDMA wireless telephone system, all users transmit in the same bandwidth at the same time and each user""s transmission contributes to the interference experienced by all other users. A power control process is used to adjust the transmit power to achieve a minimum desired signal quality at the receiver. The interference contribution experienced by other users is minimized because the transmit power to each user is minimized. Because the interference level is minimized, the number of user""s that can simultaneously communicate over the channel is maximized.
A typical CDMA communication system uses closed loop power control to help alleviate the problems associated with a non-ideal link from the transmitter to the receiver. A closed loop control process is used to control transmission power on both the forward and reverse links in a CDMA system. In closed loop control, a transmission is made, a measurement is made at the receiver, and feedback is provided to the transmitter.
In closed loop power control, the receiver tracks the performance of a demodulator and calculates a metric that is based on the quality of the received signal. A typical metric used in the receiver is the received energy per bit to noise power ratio (Eb/Nt). The receiver performance may be characterized over varied Eb/Nt values such that a probability of detection is known for a given Eb/Nt value. Alternatively, the receiver may estimate an actual signal error rate based on its ability to correctly recover the received signal and may use this error rate as the received signal metric. Then, the receiver communicates a power control signal back to the transmitter that is based in part on the received signal metric. For example, when the received Eb/Nt is high or the demodulated signal error rate is low, the receiver communicates power control information to the transmitter that allows the transmitter to decrease the transmitted power to the particular receiver. Conversely, when the received Eb/Nt is low, or the recovered signal error rate is high, the receiver communicates a power control signal back to the transmitter requesting an increase in the transmitted power to the particular receiver.
The use of DTX compounds the problem of non-ideal signal recovery. In addition to the problem of not being able to correctly demodulate transmitted data, the receiver must also determine whether or not data was actually transmitted. The receiver uses a DTX detection algorithm to determine whether or not data was transmitted.
Errors in the DTX detection algorithm of a single receiver potentially affect the entire communication link. The DTX detection algorithm of a particular receiver may inaccurately determine that DTX occurred when in reality the received signal was too degraded to allow accurate recovery. In this instance, the receiver does not communicate a request to the transmitter to increase the transmit power because the receiver has determined that it accurately decoded a DTX occurrence. The performance of the particular receiver then is degraded over optimal since a request to increase the transmit power should have been sent. The converse situation has greater adverse effects on the communication link. In this situation, the DTX detection algorithm of a particular receiver determines that it was unable to recover the transmitted signal when, in reality, the transmitter did not transmit a signal and the receiver should have indicated DTX. In this situation, the receiver communicates a request to the transmitter to increase the transmit power level to the particular receiver. This results in a transmit power level that is higher than the receiver requires to achieve a given received signal quality. The excess power increases the interference level seen by all other users and thus decreases the capacity that the communication link is able to carry.
Thus, it can be seen that there is a need for accurate DTX detection in receivers that operate in DTX capable channels. The DTX detection components need to be physically small enough to fit into portable devices, such as mobile telephones, and should consume minimal resources within the receiver.
Novel techniques are disclosed for detecting discontinuous transmission (DTX) over a communication channel. A received data frame is characterized as one of a Good frame, Erasure, or DTX. If a Good frame is not initially detected, a multi-dimensional quality metric is used to characterize the received frame as either an Erasure or DTX. A two dimensional quality metric may be generated using energy per bit to noise power ratio as a first dimension and re-encoded symbol error count as a second dimension. Alternatively, normalized re-encoded symbol energy may be used as the second dimension of the quality metric. The multi-dimensional quality metric may be generated using a polynomial having a number of variables equal to the number of dimensions. More than one multi-dimensional quality metric may be used to correspond to more than one processing mode in a receiver. The value of the multi-dimensional quality metric is computed based on the values of each dimension. The computed value is compared against a predetermined threshold and the frame is characterized based on the results of the comparison. The communication channel may be a CDMA wireless communication channel capable of DTX and the device may be a base station, base station controller, or mobile station.